Split into per-article modules and add the method-security module

Moves the existing virtual-thread/context-propagation project into
context-propagation/ and adds method-security/ for the Spring Security 7
method-security article: nine runnable demos, fourteen assertions, and every
transcript the article quotes, regenerated by scripts/run-all.sh.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RSrsDSRKVsY588yFiMJMo9

context-propagation/scripts/run-all.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Compiles and runs all seven demos plus the JUnit contract test suite, regenerating
+# docs/output/*.txt. Requires JDK 25 (StructuredTaskScope is a preview API through JDK 25 /
+# JEP 505).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+mvn -q dependency:build-classpath -Dmdep.outputFile=cp.txt
+CP=$(cat cp.txt)
+
+rm -rf target/classes
+mkdir -p target/classes docs/output
+javac --release 25 --enable-preview -cp "$CP" -d target/classes $(find src/main -name '*.java')
+
+for demo in Demo1PlainThreadLocal Demo2AsyncVirtualThreads Demo3StructuredConcurrency \
+            Demo4ExecutorWrapping Demo5ReactiveContext Demo6ScheduledSystemIdentity \
+            Demo7ServletFilterPersistence; do
+  num=$(echo "$demo" | grep -o '^Demo[0-9]*' | grep -o '[0-9]*')
+  out="docs/output/demo${num}.txt"
+  echo "Running $demo -> $out"
+  java --enable-preview -cp "target/classes:$CP" "com.ankurm.vt.$demo" \
+    | grep -v '^Picked up JAVA_TOOL_OPTIONS' | tee "$out"
+done
+
+echo "Running JUnit contract test suite -> docs/output/tests.txt"
+mvn -q test > /tmp/mvn-test-raw.txt 2>&1 || true
+{
+  echo "mvn test -- SecurityContextPropagationContractTest (10 tests pinning the claims each demo prints above)"
+  echo
+  cat target/surefire-reports/com.ankurm.vt.SecurityContextPropagationContractTest.txt
+} > docs/output/tests.txt
+cat docs/output/tests.txt
+rm -f /tmp/mvn-test-raw.txt